## 32.8. Regular Expressions: Specifying a Range of Characters with [...]

If you want to match specific characters, you can use square brackets, `[ ]`, to identify the exact characters you are searching for. The pattern that will match any line of text that contains exactly one digit is `^[0123456789]$`. This is longer than it has to be. You can use the hyphen between two characters to specify a range: `^[0-9]$`. You can intermix explicit characters with character ranges. This pattern will match a single character that is a letter, digit, or underscore: `[A-Za-z0-9_]`. Character sets can be combined by placing them next to one another. If you wanted to search for a word that:

- started with an uppercase T,

- was the first word on a line,

- had a lowercase letter as its second letter,

- was three letters long (followed by a space character (``)), and

- had a lowercase vowel as its third letter,

the regular expression would be:

>     ^T[a-z][aeiou]

To be specific: a range is a contiguous series of characters, from low to high, in the ASCII character set.[101] For example, `[z-a]` is not a range because it's backwards. The range `[A-z]` matches both uppercase and lowercase letters, but it also matches the six characters that fall between uppercase and lowercase letters in the ASCII chart: `[`, `\`, `]`, `^`, `_`, and `'`.

> [101]Some languages, notably Java and Perl, do support Unicode regular expressions, but as Unicode generally subsumes the ASCII 7-bit character set, regular expressions written for ASCII will work as well.

-- BB

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